1. Field of the Invention
The present invention relates to RF and microwave filters, and more particularly to simplifying the filter design and prototype processes.
2. Description of the Related Art
Presently, RF and microwave filters (RFMF) are used extensively in most communication devices, radar and RF/microwave systems. They are used to create the desired RF or microwave output signal--free of unwanted spurious signals and with the proper output characteristics. RF/microwave telecommunication equipment manufacturers use millions of these filters per year. These filters are used in cellular basestations, satellite communication systems and microwave communication links to name a few typical applications. RFMF components are either made internally by the equipment manufacturer or procured externally. Most of the time these filters are procured because the required filter specifications are often difficult to manufacture, and thus many companies specialize in making RFMF designs. Such filters range in frequency from .about.5 MHz to 100 GHz, usually in the 200 MHz to 4 GHz range. Some companies focus a great deal into military systems while, others focus on commercial pro/ducts. Many different types of filters are made by these companies including dielectric filters (using conductivity coated ceramic blocks), LC filters, comb filters, notch filters, helical filters, coupled cavity filters and the like. Most companies make custom filters but have a catalog of standard filters. Some companies, but not many, have many standard filters. Most companies and their distributors do not stock standard filters.
Engineers using filters usually write their own specifications so that a company can submit a design proposal. Some companies have software to help engineers specify and define filters. If the engineer likes the proposal they request or buy samples from the manufacturers they prefer. This process generally takes four to twelve weeks. When the engineer gets the RFMF, he tests it and sometimes makes changes to the requirement and the process continues, thus sometimes the system requirements change as the design progresses. Spurious signals become apparent and they have to be reduced, e.g., by RF emission testing (per FCC criteria) which may require different filter characteristics etc. Accordingly the process may require about one to six months to complete. If the filters, however, are not too difficult to make and the cost is a major consideration the filters are sometimes made internally using standard inductors and capacitors, or by on board techniques such as microstrip coupled lines. Some companies sell variable filters which can tune over a wide range of frequencies, however these filters are expensive, large, connectorized and thus for most situations can not be used in prototype systems.
There are numerous shortcomings associated with existing filter design practices, such as design time, lack of flexibility, difficulty in communicating needs, and various difficulties associated with simulating and building prototypes. First, as discussed above this process can take up to six months or more to build and test a desired filter design. Alternatively, the circuit designer may use commercially available parts, but must then contend with the attendant lack of flexibility and availability of a particular filter characteristic. Thus the engineer must modify their circuit design to accommodate the use of the limited number of readily available filters. To this end, one must take what is given and can not change many times because of the cost and time constraints associated with standard and custom filters.
Secondly, many times difficulty arises in communicating the engineers exact filter requirements because the systems are often so complex that it is difficult to communicate every specification which is required. For example, the filter manufacturing company might build the filter for a 50 ohm load but what is actually needed is a different impedance. Often the engineer does not know exactly what he really wants until the system is put together. As a result the filter maybe incorrectly specified.
Furthermore, difficulty occurs in simulating a circuit or system because of the lack of exact information on the filter. Many other components such as amplifiers, attenuators, switches are well characterized by the manufacturers and their S-parameters can be put into computer programs that simulate the circuit or system accurately. Filters also present a design problem because many times the engineer does not know the exact response or impedance requirement until the engineer receives the actual part from which components are characterized to extract the S-parameters. Some system simulators only require the passband, rejection and group delay of the filter, but more detailed circuit simulators require S-parameters or an equivalent circuit.
Finally, filters are often the rate determining step when building a RF/microwave system and many times present the most significant difficulty to building the system quickly. Other components such as amplifiers, attenuators, switches and mixers are broadband such that standard product will be available in short notice from many manufacturers and distributors. Filters are generally not broadband and are by definition frequency specific. With the exception of some standard telecommunications frequency filters, most are typically not held in stock because of their specialized nature. Many times engineers desire to modify a standard filter's characteristics such as bandwidth, rejection, ripple, impedance, etc.
Numerous problems are associated with building experimental high frequency filters on test boards. They include a lack of performance due to low Q components and board type restrictions, tuning requirements, as well as the time required to build and test the filter design. Generally a test board must be created, components must be characterized at required frequencies, and finally the filter must then be tested and tuned.